Associating SCFs with Element Points

Associating SCFs with Element Points - Ultramarine.com Associating SCFs with Element Points

In computing fatigue in elements, the element SCFs are used. For elements there are four ways to define them: on a &DEFAULT command, on the class definition command, on the element definition command, or in some cases from the tubular joint SCFs. In any case failure to include a definition results in the last definition being used; i.e.

For intermediate fatigue points between tubular sections, an SCF will be computed based on the algorithm specified on the -IN_SCF option on the &PARAMETER command,
For intermediate fatigue points between tubular and conical sections, an SCF will be computed as specified in API RP2A or DNV (they are the same). MOSES will also check for two tubes that have no braces and will compute an inline SCF as if the change in section was internal to the beam itself. For other cases, the -SCF specified on the element command is used, If no -SCF is specified on the element command, then that specified on the class definition is used, If no -SCF is specified on the class definition, then that defined on the &DEFAULT is used.
In the case where one of the ends of a tubular element is part of a tubular joint, then a part of the joint SCF will be used for the SCF of the element at that end.

To define the "inline" SCF between two tubular sections, then the



     -IN_SCF, TYPE

option of &PARAMETER is used. There type must be either DNV or a number. The number is the exponent, e, used in computing the SCF according to:

     SCF = 1 + A * B
 
     A   = 3 ( T2 - T1 )/ T1
 
     B   = T1**e / ( T1**e + T2**e)

Where T1 and T2 are the tubular thickness. If DNV is specified a value of 2.5 will be used for e. Here T2 is greater than T1.

For the &DEFAULT command, the default SN curves at various points on and element are defined with the option



     -SCF, TYPE(1), SCF(1), ....

The -SCF option defines default stress concentration factors for different types of section. Here, TYPE(i) defines a section type and must be either: BOX, PRI, BU, W, M, S, HP, WT, MT, ST, L, C, MC, WBOX, DL, LLEG, or PLATE. The values SCF(i) are the stress concentration factor for the corresponding type. Please notice that one cannot define a default SCF for a tube. This would defeat the normal computation of SCF's for joint fatigue. If you have a tube, want to do beam fatigue, and want an SCF other than 1, you need to manually do it on the command which defines the beam.

For class definition commands, we have the option:



     -SCF SCF_BEG, SCF_END

Here, SCF_BEG is the SCF which will be used at the beginning of the segment and SCF_END is the curve used at the end of the segment. If one uses two SCFs for each segment, the one in the middle will be defined twice. If this is done, the second definition will actually be used; i.e. the first SCF specified will, in fact, be used for the beginning of the segment. Also, SCF_END needs to be specified only on the last segment.

The SCF definition for elements is accomplished with the -SCF option which is followed by from one to eight numbers. If there is no SCF option, then the default SCF defined for this class will be used. Here, -SCFA will change the values at the first end of the beam -SCFB will change them at the other end, and -SCF1 will change them at the intersection between the first segment and the second, etc. If -SCF is used, then the stress concentration factor will be the same at all fatigue points. For the ends of the beam, the form of this option is:



     -SCFi, VALUES(1), ... VALUES(n)

One can input 0, 1, 3, 4, 8, or 24 values. Zero values sets the SCFs to the default values. If 1 values is input, then it will be used as the SCF for axial and for strong and weak axis bending. If three values are input then they are the axial, strong axis, and weak axis binding SCFs.

The remaining numbers (4, 8, or 24) are used to override the computed SCFs when the beam is part of a tubular joint. The first four values here define the SCFs at the brace at the saddle, crown, in plane bending and out of plane bending. If only four values are given, then these will be repeated for all three load classifications and for the chord. With eight values, the second four values define the SCFs for the chord and the brace/chord values will be used for all joint classifications. If twenty four values are specified, the first twelve are for the chord in K joints, T&Y joints and X joints, and the last twelve are the corresponding values for the chord. K, T&Y, and X. (See section on Associating SCFs with Tubular Joints for more information).

Also, the "joint SCFs" are used when computing beam fatigue. Here, the maximum of the crown and saddle SCFs and the maximum inplane and out of plane SCFs for all classifications will be used at the end when computing beam fatigue. This will result in beam fatigue which will be a bit less that that computed at the same end by joint fatigue, but it is not excessively conservative for slamming fatigue.